The present invention relates generally to communications, and more particularly to a system and method for analog reconstruction of a digital signal.
Filterbanks have been employed in many applications such as transmultiplexers, audio/video compression, and adaptive filtering. A typical reconstruction filterbank (e.g., near perfect reconstruction filterbank) is designed to filter a wide band digital signal comprised of a plurality of subbands or subchannels into the subbands or subchannels (e.g., channelize the wideband signal), process the subbands or subchannels (e.g., compress/decompress), and then reconstruct the subbands or subchannels (e.g., reconstructing or synthesizing the subbands) into a wide band digital signal with an attempt to minimize alias distortion. Oversampled filterbanks are widely employed to reduce the computational complexity of signal processing algorithms, such as subband adaptive filtering techniques utilized in many audio/video compression techniques. The signal channelizing and synthesizing is typical performed employing algorithms executed on a digital signal processor.
Filterbanks are employed in wireless applications for separating channels of a wideband signal and for reconstructing the channels at similar or different frequency bands into a wideband signal prior to transmission of the wideband signal over a wireless link. After the signal channeling and signal combining algorithms are performed, the wideband signal is typically transmitted over a radio frequency (RF) wireless link to one or more other radio devices (e.g., user terminals). The transmission frequency of the RF wireless link is at a substantially higher frequency than the processing frequency of the wideband signal. Therefore, the wideband signal is converted from the digital domain to the analog domain and mixed with an intermediate frequency source to provide a wideband analog signal at the higher transmission frequency.
Conventional methods of converting a digital complex baseband output of a reconstruction filterbank into the analog domain employ a digital-to-analog converter (DAC). The output of the DAC is then provided to an analog mixer to frequency shift the analog signal to an intermediate frequency. The analog signal at the intermediate frequency can then be provided to an RF carrier at a transmission frequency or upmixed again to the RF transmission frequency. However, the frequency shifting of the analog signal adds undesired distortion to the analog signal. Additionally, a DAC is designed to have a sampling rate that provides a frequency response having a central flat region (i.e., accurate bandwidth region of the DAC) and outer non-flat regions (i.e., outside the accurate bandwidth region of the DAC). In typical systems, the digital signal provided at the input of the DAC is at a frequency that is in the non-flat frequency response region of the DAC, since the frequency of the signal is higher or lower than the accurate bandwidth region of the DAC. This causes alias distortion of the wideband output signal. The alias distortion needs to be removed prior to signal transmission.
Commonly, an xe2x80x9cInverse Sincxe2x80x9d compensation function is performed on the wideband output signal to eliminate distortion caused by the non-flat frequency response in the higher operating frequency ranges of the DAC. However, the xe2x80x9cInverse Sincxe2x80x9d compensation function increases the complexity of the system.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention includes a system and method for efficient analog reconstruction of a digital signal. The digital signal can be a complex baseband signal that is sourced from a near perfect reconstruction filterbank (NPRFB). The digital signal is upsampled to provide a plurality of spectral images or copies of the complex baseband signal, such that one of the spectral images is centered at an intermediate frequency (IF). The plurality of spectral images is then filtered to provide a spectral image centered at IF. The real portion of the spectral image centered at IF is extracted and provided to a high speed digital-to-analog converter (DAC) to convert the spectral image centered at IF to an IF analog signal. The IF analog signal can then be modulated or further mixed with a RF frequency transmission signal for RF transmission with low distortion. The spectral image centered at IF is input into the DAC in the flat frequency response region of the DAC providing an IF analog signal with low distortion. This low distortion scheme is efficient in terms of digital computation and requires no extra analog hardware.
In one aspect of the present invention, a digital complex baseband signal is upsampled in the digital domain to provide a plurality of spectral images centered at different frequencies. The upsampled signal is interpolated with a complex tap digital filter, such the spectral image centered at IF is extracted and the other spectral images are rejected. In another aspect of the present invention, a real filter is employed to filter off the spectral image centered at complex baseband and a multiplier mixed with the spectral image centered at complex baseband to shift the spectral image to be centered at IF. In either aspect of the invention, the complexity of the filter can be reduced by providing an oversampled signal, for example, from a NPFRB. Since the reconstructed signal is bandlimited and oversampled, the upsampling of the images result in further separation in frequency allowing a filter with more gradual rolloff. Once upsampled and filtered, the signal can be input in the flat frequency response range of a high speed DAC.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.